Optical fibers are being used in a variety of biosensor applications. For example, as discussed in U.S. Pat. No. 5,494,798, an optical fiber may be used without cladding to exploit the evanescent field present immediately outside the fiber/air interface. If a monoclonal or polyclonal antibody is attached to the exposed surface of the bare fiber, the evanescent field envelopes the molecule. But since there is little or no absorption or other phenomena to alter the amount of the light carried by the fiber, no attenuation or detectable characteristics are developed.
However, when an appropriately labeled antigen is attached to the antibody, the evanescent field can cause the antigen to fluoresce, resulting in an optically detectable characteristic such as a reduction in light intensity or fluorescence. Alternatively, by first binding the antigen, the sensor can used to detect unknown targets, including toxins or immunogenic agents.
Whereas previous fiber-optic evanescent-wave sensors utilized multi-mode fibers, the '798 patent improved on the technique by employing a pair of single-mode optical fibers in a coupler arrangement. Light is introduced into one of the fibers to produce an evanescent region surrounding the coupling area, and the magnitude of light emitted from the pair of fibers is compared for detection purposes.
FIG. 1, taken from the '798 patent, shows the overall fiber optic system generally at 10. Light from laser diode 14 is inserted into a first leg 17 of a fiber optic coupler 18, and exits on the same fiber at 19 (input channel). A second fiber 20 provides an output channel for light from the first leg 17. A first photo diode detector 21 is connected to the input channel and a second photo diode detector 22 is connected to the output channel.
Each detector feeds its own transimpedance amplifier. The outputs of the transimpedance amplifiers 23, 24 are applied to A/D converters 25 and 26 which provide digital electrical signals along wires 27 and 28 to an instrumentation board 29. The instrumentation board 29 is then connected to a personal computer 30 which provides outputs to a printer or a monitor.
The finished probe includes the coupler and attached antibodies, which yields a baseline ratio for the sensor. The finished probe is then exposed to a material of interest, and the ratio of the light through the two sides of the coupler changes as a function of the way in which the target attaches. That is, the localized index of refraction at the coupling region and the determination of the ratio is a function of the binding in the coupler region.
In terms of the coupler itself, existing designs use off-the-shelf components intended for multiplexers and demultiplexers in telecommunications applications. Corning, for instance, makes these couplers by twisting together two or more 1300-nm, single-mode type 9-125 optical fibers, heating up the twisted area and pulling the ends apart to create a necked-down, nearly fused union. The number of fibers and other factors such as the proportion of each fiber in the twisted region determines the coupling ratio.